Technical Abstract:
The dominance of the corn-soybean rotation, which covers 40 million hectares in the north central USA, is likely to continue due to such factors as government programs, capital investment in equipment, and availability of markets. Nitrate losses are often in the range of 20 to 100 kg N/ha, even when best management practices are used for fertilizer or manure N management in this system. Cover cropping is one tool that could be used to reduce water quality degradation by this rotation because it increases the amount of time the land is covered with growing vegetation. This experiment was conducted in southern Minnesota on a Webster clay loam (Typic Endoaquoll) soil. Winter rye (Secale cereale) was planted in the fall of 1998 and 1999. Weather conditions between fall 1998 and spring 1999 resulted in optimum growing conditions for the rye. During 1999, the rye cover crop reduced water loss from subsurface tile lines with cover crop treatments by 23% and lowered nitrate-N loss by 50% compared with the corn soybean rotation without the cover crop. Weather conditions during the fall of 1999 and spring of 2000 were dry, limiting rye growth. Water loss from subsurface tile lines occurred for a brief period in late spring 2000. Treatments with the cover crop had less drainage (0.05 versus 3.7 m3), resulting in 40% less nitrate-N loss from subsurface tile lines. Between fall 1998 and spring 1999, residual soil nitrate to 150 cm decreased by 9% following corn without the cover crop compared to a 59% decrease with the cover crop treatment. Presence or absence of the cover crop did not affect subsequent soybean yield (mean = 2.7 Mg/ha). Nitrate loss from subsurface drainage between 1999 and 2001 during contrasting environmental conditions will be presented in the context of the historical weather record.